EP0471190A2 - Method for preparation of phosphorous acid - Google Patents

Abstract

In a continuous process for the preparation of phosphorous acid with simultaneous production of hydrochloric acid, a reaction zone is provided which is in open communication with a first scrubbing zone. A portion of the reaction product is recycled from the bottom of the reaction zone into the top of the scrubbing zone and phosphorous acid is removed from the circulation. Hydrogen chloride gas escaping from the top of the first scrubbing zone is brought into contact, in a cooled, second scrubbing zone, with an overstoichiometric amount of water. Hydrochloric acid is obtained which is passed as starting material into the reaction zone or into the top of the first scrubbing zone. The external cooling of the second scrubbing zone is controlled such that the temperature in the reaction zone is kept constant at a desired value between 90 DEG and 130 DEG C. without adding any external cooling or heating energy. Pure hydrochloric acid is recovered in an absorption zone.

Description

• a) hohe freiwerdende Reaktionswärme
  Die Reaktion läßt sich von der Wärmebilanz her in zwei Teilreaktionen zerlegen:
  PCl₃ (flüssig) + 3 H₂O (flüssig) → H₃PO₃ (flüssig) + 3 HCl (gasförmig). Diese Teilreaktion ist schwach exotherm.
  
  
  
          HCl (gasförmig) + x H₂O (flüssig) → HCl (aq.)
  
  
  
  Die Auflösung gasförmigen Chlorwasserstoffs in Wasser ist eine stark exotherme Reaktion.
  Die auftretenden hohen Temperaturen führen zu unerwünschten Nebenreaktionen. Da der größere Teil der freiwerdenden Wärmemenge durch die Lösungswärme des Chlorwasserstoffgases bedingt ist, ist in der DE-AS 1 042 551 vorgeschlagen worden, die Hydrolyse von PCl₃ mit konzentrierter, rauchender Salzsäure durchzuführen. Die bei der Hydrolyse des PCl₃ freiwerdende Wärmemenge sollte die zur Entfernung des auf diese Weise zugeführten Chlorwasserstoffgases verbrauchte Wärmemenge kompensieren. Tatsächlich wird aber - entgegen diesbezüglichen Aussagen in der DE-AS 1 042 551 - die Gesamtreaktion stark endotherm, so daß ständig Energie zugeführt werden muß. Gemäß der DE-AS 1 042 551 erfolgt die Hydrolyse im übrigen bei Temperaturen unter 25 °C. Das Verfahren kann auch kontinuierlich durchgeführt werden.
• b) schlechte Mischbarkeit der Reaktanten (Unterschichtung und lokale Überhitzung)
  Aufgrund der Dichteunterschiede von PCl₃ und Wasser bzw. verdünnter Phosphoriger Säure besteht stets die Gefahr der Unterschichtung mit plötzlicher unkontrollierter Abreaktion. Gemäß FR-Patent 1.214.039 wird dieser Gefahr dadurch vorgebeugt, daß die Reaktion bei einer Temperatur oberhalb des Siedepunkts von PCl₃ (76 °C) durchgeführt wird. Dennoch wird häufig aufgrund örtlicher höherer Konzentration an PCl₃ die Bildung von Zersetzungsprodukten beobachtet. Gemäß FR-1.214.039 wird PCl₃ in eine Reaktionskolonne (Füllkörperkolonne) bei Temperaturen von 80 bis 100 °C eingeleitet. Der Reaktionskolonne ist eine zweite, beheizte Füllkörperkolonne aufgesetzt, durch die von oben her eine Mischung aus umgepumpten Reaktionsgemisch und Wasser herabrieselt. Am Fuß der Reaktionskolonne wird ständig rohes Reaktionsprodukt (H₃PO₃) abgezogen und teils im Kreislauf auf den Kopf der oberen Füllkörperkolonne gepumpt, teils auf reine H₃PO₃ aufgearbeitet. Das entstehende HCl-Gas wird am Kopf der oberen Füllkörperkolonne abgezogen. Eine stöchiometrische Menge Wasser wird dem umgepumpten Reaktionsgemisch kurz vor Eintritt in die obere Füllkörperkolonne zugegeben.
• c) Verluste an PCl₃ über die Gasphase
  Bei den technisch angewandten Verfahren wird stets ein Teil des eingesetzten PCl₃ von dem bei der Reaktion entstehenden Chlorwasserstoffgas mitgerissen und gelangt so in die HCl-Absorptionsstufe. Neben der hierdurch bedingten Verminderung der Wirtschaftlichkeit wird durch diese Verunreinigung der Verkaufswert der gewonnenen konzentrierten Salzsäure stark herabgesetzt.
  Die US-2,684,286 beschreibt ein kontinuierliches Verfahren zur Herstellung von H₃PO₃ durch Hydrolyse von PCl₃ mit einem 20 bis 40 %igen stöchiometrischen Überschuß an Wasser bei Temperaturen unterhalb von 76 °C, vorzugsweise bei 40 bis 60 °C. Die Ausgangsstoffe werden hierbei in eine Vorlage eines Gemischs aus H₃PO₃ und Salzsäure eingeleitet. Das über Kopf des Reaktionsgefäßes entweichende Chlorwasserstoffgas wird zurückgewonnen, enthält aber üblicherweise noch etwas PCl₃.

It is known to produce phosphorous acid (H₃PO₃, recently also called phosphonic acid) by hydrolysis of phosphorus trichloride (PCl₃). However, difficulties arise during technical implementation due to the following issues:

• a) high released heat of reaction
  The heat balance of the reaction can be broken down into two sub-reactions:
  PCl₃ (liquid) + 3 H₂O (liquid) → H₃PO₃ (liquid) + 3 HCl (gaseous). This partial reaction is slightly exothermic.
  
  
  
  HCl (gaseous) + x H₂O (liquid) → HCl (aq.)
  
  
  
  Dissolving gaseous hydrogen chloride in water is a highly exothermic reaction.
  The high temperatures that occur lead to undesirable side reactions. Since the greater part of the amount of heat released is due to the heat of solution of the hydrogen chloride gas, it has been proposed in DE-AS 1 042 551 to carry out the hydrolysis of PCl₃ with concentrated, smoking hydrochloric acid. The amount of heat released during the hydrolysis of the PCl₃ should compensate for the amount of heat consumed to remove the hydrogen chloride gas supplied in this way. In fact, contrary to what is stated in DE-AS 1 042 551, the overall reaction becomes strongly endothermic, so that energy must be supplied continuously. According to DE-AS 1 042 551, the hydrolysis also takes place at temperatures below 25 ° C. The process can also be carried out continuously.
• b) poor miscibility of the reactants (underlayering and local overheating)
  Due to the density differences of PCl₃ and water or dilute phosphorous acid, there is always the risk of under-layering with a sudden, uncontrolled reaction. According to FR Patent 1,214,039, this danger is prevented by carrying out the reaction at a temperature above the boiling point of PCl₃ (76 ° C). Nevertheless, the formation of decomposition products is often observed due to the locally higher concentration of PCl₃. According to FR-1.214.039 PCl₃ is introduced into a reaction column (packed column) at temperatures from 80 to 100 ° C. The reaction column is fitted with a second, heated packed column, through which a mixture of pumped-around reaction mixture and water trickles down. At the foot of the reaction column, crude reaction product (H₃PO₃) is continuously drawn off and partly pumped into the circuit at the top of the upper packed column, partly worked up on pure H₃PO₃. The resulting HCl gas is drawn off at the top of the upper packed column. A stoichiometric amount of water is added to the pumped reaction mixture shortly before entry into the upper packed column.
• c) losses of PCl₃ over the gas phase
  In the technically applied process, part of the PCl₃ used is always entrained by the hydrogen chloride gas formed in the reaction and thus reaches the HCl absorption stage. In addition to the resulting reduction in economic efficiency, this contamination greatly reduces the sales value of the concentrated hydrochloric acid obtained.
  The US-2,684,286 describes a continuous process for the preparation of H₃PO₃ by hydrolysis of PCl₃ with a 20 to 40% stoichiometric excess of water at temperatures below 76 ° C, preferably at 40 to 60 ° C. The starting materials are introduced into a template of a mixture of H₃PO₃ and hydrochloric acid. The escaping over the top of the reaction vessel hydrogen chloride gas is recovered, but usually contains some PCl₃.

It is the object of the present invention to avoid the difficulties encountered in the known methods and to provide a solution to the problems described.

The invention relates to a continuous process for the production of phosphorous acid with simultaneous recovery of hydrochloric acid by hydrolysis of phosphorus trichloride with aqueous hydrochloric acid at elevated temperature in a reaction zone which is charged with the reaction products and which is in open communication with a first washing zone arranged above it, at the top thereof Part of the reaction product is pumped from the bottom of the reaction zone in the circuit, while hydrogen chloride gas is withdrawn from the circuit at the top of the first washing zone and phosphorous acid, which is characterized in that for the preparation of a 60 to 110% by weight phosphorous acid, Calculated as H₃PO₃ equivalent, while simultaneously obtaining a pure, 20.2 to 42 wt .-% hydrochloric acid, the hydrogen chloride gas escaping at the top of the first washing zone in a cooled, second washing zone with a, calculated on the amount P fed into the reaction zone phosphorus trichloride, a stoichiometric amount of water in contact and, depending on the degree of external cooling of the second washing zone to an internal temperature of 80 to 110 ° C, a 32 to 20.2 wt .-% hydrochloric acid is obtained, which is used as a starting material in the reaction zone or to the top of the first washing zone, taking into account the fact that the concentration of hydrochloric acid in the second washing zone is all the lower and consequently the temperature The lower the second washing zone is cooled from the outside - and vice versa - the higher the outside cooling of the second washing zone controls the higher the temperature in the reaction zone so that the temperature in the reaction zone remains constant at a desired value without the addition of external cooling or heating energy 90 and 130 ° C is maintained, and that from the excess, newly formed during the reaction and at the top of the second washing zone together with excess water vapor escaping undissolved escaping hydrogen chloride gas in a possibly cooled absorption zone with the addition of water, 20.2 up to 42% by weight hydrochloric acid.

• a) in der Absorptionszone eine 30 bis 36 gew.-%ige Salzsäure gewinnt,
• b) in die zweite Waschzone als Reaktionswasser eine verdünnte Phosphorige Säure mit vorzugsweise weniger als 70 Gew.-% H₃PO₃ einleitet,
• c) einen Teil des Reaktionsprodukts vom Boden der Reaktionszone im Kreislauf in deren obere Hälfte pumpt,
• d) den vom Boden der Reaktionszone im Kreislauf in deren obere Hälfte gepumpten Teil des Reaktionsprodukts unterhalb der Oberfläche des Reaktionsproduktes tangential zur Wandung der Reaktionszone einleitet,
• e) die aus dem Kreislauf des Reaktionsprodukts abgezogene Phosphorige Säure durch Abstreifen oder Aufkonzentrieren unter vermindertem Druck von Chlorwasserstoff und Wasser befreit.

In addition, the method of the invention can be preferred and optionally characterized in that

• a) a 30 to 36% by weight hydrochloric acid is obtained in the absorption zone,
• b) a dilute phosphorous acid with preferably less than 70% by weight of H₃PO₃ is introduced into the second washing zone as reaction water,
• c) pumps part of the reaction product from the bottom of the reaction zone into the upper half of the circuit,
• d) introduces the part of the reaction product pumped from the bottom of the reaction zone into the upper half of the circuit below the surface of the reaction product tangentially to the wall of the reaction zone,
• e) the phosphorous acid withdrawn from the circuit of the reaction product is freed from hydrogen chloride and water by stripping or concentrating under reduced pressure.

Under a more than 100 wt .-% phosphorous acid, calculated as H₃PO₃ equivalent, is to be understood a phosphorous acid that contains significant proportions of pyrophosphorous acid (H₄P₂O₅, previously also diphosphorous acid, now called diphosphonic acid) and on addition (at least ) stoichiometric amounts of water, at room temperature or faster at elevated temperature, a (at most) 100 wt .-% phosphorous acid (H₃PO₃) provides. For example, 100 g of a 110 wt .-% phosphorous acid consist of only 19 g H₃PO₃ in addition to 81 g H₄P₂O₅. On addition of 10 g H₂O - corresponding to the stoichiometric equivalent amount - 91 g H₃PO₃ arise from the 81 g H₄P₂O₅ and you get a total of 110 g of a 100 wt .-% H₃PO₃.

• 1. Es kann Phosphorige Säure, ggf. im Gemisch mit Pyrophosphoriger Säure, in einem weiten Konzentrationsbereich, nämlich von 60 bis 110 Gew.-%, hergestellt werden;
• 2. durch eine Variation der Konzentration der zur Hydrolyse eingesetzten wäßrigen Salzsäure zwischen 20,2 und 32 Gew.-% wird eine gute Steuerung der Gesamt-Wärmebilanz im Reaktionsgefäß ermöglicht;
• 3. es wird eine sehr gute Durchmischung des Reaktorinhalts gewährleistet;
• 4. es werden keine Verluste an PCl₃ festgestellt, so daß aus dem bei der Hydrolyse von PCl₃ entstehenden reinen HCl-Gas eine reine konzentrierte 20,2 bis 42 gew.-%ige Salzsäure hergestellt werden kann.

The following advantages result when carrying out the method of the invention:

• 1. Phosphorous acid, possibly in a mixture with pyrophosphorous acid, can be produced in a wide concentration range, namely from 60 to 110% by weight;
• 2. By varying the concentration of the aqueous hydrochloric acid used for the hydrolysis between 20.2 and 32% by weight, good control of the overall heat balance in the reaction vessel is made possible;
• 3. very good mixing of the reactor contents is ensured;
• 4. no losses of PCl₃ are found, so that a pure concentrated 20.2 to 42% by weight hydrochloric acid can be produced from the pure HCl gas resulting from the hydrolysis of PCl₃.

The method of the invention is explained in more detail below with reference to the accompanying drawing:
In the upper half of the heatable reactor 1, in which there is already a phosphorous acid saturated with HCl gas of the desired concentration between 60 and 110% by weight (calculated as H₃PO₃ equivalent), are below the surface of the phosphorous acid submitted simultaneously via line 2 PCl₃ and via line 3 a 20.2 to 32 wt .-% hydrochloric acid in approximately stoichiometric amounts. The temperature in reactor 1 is kept at 90 to 130 ° C. The lower the concentration and the higher the temperature of the phosphorous acid, the higher the turnover of PCl₃. However, it is at least 75%.

The resulting HCl gas and usually entrained residual amounts of PCl₃ escape through the top of the reactor 1 and reach the heatable first washing column 4, which is filled with packing. At the bottom of the reactor 1, the reaction mixture is continuously drawn off and pumped to the top of the washing column 4 in a circuit via the heated line 5 using the pump 6. In this way, the remaining amounts of PCl₃ are hydrolyzed in the washing column 4 to H₃PO₃ and flushed back into the reactor 1.

Through the heated branch line 7, the reaction mixture is continuously removed from line 5 and circulated into the upper half of the reactor 1, which ensures thorough mixing of the reactor contents, which can be improved further if the inlet connection of line 7 into the reactor 1 laid below the surface of the reaction product tangential to the reactor wall. The required thorough mixing can also be achieved by installing and operating a stirrer in the reactor 1.

Through the heated branch line 8 reaction mixture from line 5 is continuously withdrawn and worked up to pure phosphorous acid of a desired concentration between 60 and 110 wt .-%, calculated as H₃PO₃ equivalent. The separation of water and HCl is preferably carried out by stripping with inert gas or under reduced pressure.

The HCl gas washed in the washing column 4 flows via line 9 into the second washing column 10, to which cooling water can be applied. The washing column 10 is charged from line 11 with a, calculated on the amount introduced into the reactor 1 PCl₃, over-stoichiometric amount of water. Any remaining amounts of PCl₃ in the HCl gas stream are converted into H₃PO₃ in the washing column 10. The spatial separation of the H₂O supply (in the washing column 10) from the PCl₃ supply (in the reactor 1) offers the practical quantitative removal of the PCl₃ from the reaction gases the advantage that this washing column 10 to about 80 to by varying degrees of cooling 110 ° C., the concentration of the hydrochloric acid formed here from the HCl gas stream and the water can be varied within wide limits between 20.2 and 32% by weight HCl. The hydrochloric acid concentration is about 32% by weight when cooling to about 80 ° C. and about 20.2% by weight when cooling to about 110 ° C. This hydrochloric acid is fed continuously to the reactor 1 via line 3 for the hydrolysis of the PCl₃ used. By setting a certain temperature of 80 to 110 ° C and thus a certain concentration of hydrochloric acid (20.2 to 32 wt .-%) in the washing column 10, the amount of heat released during the hydrolysis of the PCl₃ can now be removed for removal exactly compensate for the amount of heat required.

Instead of via line 3 into the reactor 1, the hydrochloric acid can also be introduced via lines 3 and 12 to the top of the first washing column 4. This is advantageous, for example, if, due to a high concentration (> 90% by weight) of the phosphorous acid in the reactor 1, the conversion of the PCl₃ in the reactor 1 is only between 75 and 85% and accordingly a larger amount of the unreacted PCl₃ rises in the washing column 4 and must be washed back from there (with hydrolysis to H₃PO₃).

Instead of pure water, a dilute aqueous solution of phosphorous acid can also be used successfully via line 11. In this way, the method of the invention can also be used to concentrate dilute phosphorous acid.

The almost pure HCl gas remaining after the second scrubbing column 10 flows together with excess water vapor via line 13 into the absorption column 14, which is filled with packing and possibly cooled and is sprinkled with water overhead via line 15. Depending on the amount of water, 20.2 to 42% by weight pure hydrochloric acid can be drawn off at the bottom of the absorption column 14 via line 16.

example 1

In the reactor 1 there is an HCl-saturated 88.7% aqueous H₃PO₃ solution at a temperature of 97 ° C. By metering in 16.0 kg / h phosphorus trichloride via line 2 into the reactor 1 and 8.0 kg / h water via line 11 into the second washing column 10 with simultaneous product removal via line 8, a uniform fill level in the reactor 1 is ensured. At the same time, with a cooling water supply of 0.07 m³ / h in the jacket of the second washing column 10, a temperature of 94 ° C. is established inside 10. After the equilibrium has been set (approx. 1 h), the heating / cooling of the reactor 1 and the first washing column 4 is switched off. After another hour or so, a new steady state has emerged; the reactor temperature is 107 - 109 ° C and remains constant for the further test period (12 h) without the addition of external cooling or heating energy. The temperature in the second washing column 10 is still 94 ° C., the concentration of the hydrochloric acid is 28.9% (line 3).

Over the entire test period, the pump (line 5) through line 8 becomes 10.8 kg / h of reaction product taken, consisting of 88.7% H₃PO₃, 8.4% H₂O and 2.9% HCl. By concentration at 140 ° C and 50 mbar, hydrogen chloride and water are largely removed and 9.65 kg / h H₃PO₃ (99%). The addition of 23.0 kg / h of H₂O through line 15 into the cooled absorption stage 14 gives 36.2 kg / h of 34.3% hydrochloric acid (line 16).

Example 2 (comparative example)

Under otherwise identical preconditions as in Example 1, a cooling water quantity of 0.40 m³ / h is used in the second washing column 10; the result is an internal temperature of 63 ° C and a hydrochloric acid concentration of 35.1%. After 1 h, the heating of the reactor 1 and the first washing column 4 is switched off. A rapid drop in the reactor temperature is observed, so that after 12 minutes an internal temperature in the reactor 1 of 90 ° C. is measured. When a temperature of 78 ° C (after 35 min) was reached, the experiment was stopped because there is a risk of under-layering with a sudden, uncontrolled reaction due to the presence of liquid phosphorus trichloride due to the difference in density of PCl₃ and water or dilute phosphoric acid.

Claims (6)

Continuous process for the preparation of phosphorous acid with simultaneous recovery of hydrochloric acid by hydrolysis of phosphorus trichloride with aqueous hydrochloric acid at elevated temperature in a reaction zone which is fed with the reaction products and which is in open communication with a first washing zone arranged thereon, on the top of which a part of the reaction product from Bottom of the reaction zone is pumped into the circuit, while hydrogen chloride gas are withdrawn from the circuit via the top of the first washing zone and phosphorous acid, characterized in that one produces H to PO₃ equivalent to produce a 60 to 110 wt .-% phosphorous acid , while simultaneously obtaining a pure, 20.2 to 42% by weight hydrochloric acid, the hydrogen chloride gas escaping at the top of the first washing zone in a cooled, second washing zone with a stoichiometric amount calculated on the amount of phosphorus trichloride fed into the reaction zone hen brings amount of water in contact and depending on the degree of external cooling of the second washing zone to an internal temperature of 80 to 110 ° C a 32 to 20.2 wt .-% hydrochloric acid is obtained, which is used as a starting material in the reaction zone or Head of the first washing zone conducts, taking into account the fact that the concentration of hydrochloric acid in the second washing zone and therefore the temperature in the reaction zone are lower, the less the second washing zone is cooled from the outside - and vice versa - , the external cooling of the second washing zone so controls that the temperature in the reaction zone is kept constant at a desired value between 90 and 130 ° C without the supply of external cooling or heating energy; and that a pure 20.2 to 42% by weight hydrochloric acid is obtained from the excess hydrogen chloride gas, which is newly formed during the reaction and escapes undissolved at the top of the second washing zone together with excess water vapor, in a possibly cooled absorption zone with the addition of water wins. Process according to Claim 1, characterized in that a 30 to 36% by weight hydrochloric acid is obtained in the absorption zone. Process according to Claim 1 or 2, characterized in that a dilute phosphorous acid with preferably less than 70% by weight H₃PO₃ is passed into the second washing zone as the water of reaction. Process according to at least one of the preceding claims, characterized in that part of the reaction product is pumped from the bottom of the reaction zone in a circuit into its upper half. Process according to Claim 4, characterized in that the part of the reaction product pumped from the bottom of the reaction zone in the upper half of the circuit is introduced below the surface of the reaction product tangentially to the wall of the reaction zone. Process according to at least one of the preceding claims, characterized in that the phosphorous acid withdrawn from the circuit of the reaction product is freed from hydrogen chloride and water by stripping or concentrating under reduced pressure.

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